1. Field of the Invention
This invention relates to an image-wise exposure apparatus, wherein a photosensitive material is exposed image-wise by using a spatial modulation device, such as a liquid crystal panel. This invention particularly relates to an image-wise exposure apparatus, wherein a picture element shifting operation is carried out such that a high resolution image may be obtained on the photosensitive material.
2. Description of the Prior Art
An image-wise exposure apparatus has heretofore been proposed, wherein recording light is modulated by a spatial modulation device, such as a liquid crystal panel, the modulated recording light is irradiated to a photosensitive material, and the photosensitive material is thereby exposed image-wise. Such an image-wise exposure apparatus is described in, for example, Japanese Unexamined Patent Publication No. 4(1992)-7542. Basically, the spatial modulation device comprises a plurality of picture elements (in the cases of the liquid crystal panel, a plurality of liquid crystal cells), the transmittances or the reflectivities of which can be changed, and which are located in the form of a two-dimensional array.
Also, as a spatial modulation device suitable for such an image-wise exposure apparatus, a mirror array device is known. The mirror array device comprises a plurality of small mirrors, which are located in the form of a two-dimensional array, and a driving means for changing the orientations of the respective small mirrors independently. The orientation of each small mirror of the mirror array device is changed such that light incident upon the small mirror may be selectively reflected toward one of two directions.
With the mirror array device described above, in cases where the driving means of the mirror array device is controlled in accordance with an image signal, the light impinging upon a predetermined projection surface via a small mirror can be modulated for each small mirror, and the image can be projected onto the projection surface. Also, in cases where a photosensitive material is located along the projection surface, the photosensitive material can be exposed image-wise. In such cases, the amount of light impinging upon the photosensitive material can be controlled for each small mirror by, for example, carrying out pulse width modulation of the on time of each small mirror (i.e., the time during which each small mirror is set in the orientation that causes the light to impinge upon the photosensitive material) within a frame period. In this manner, a gradation image can be formed on the photosensitive material.
In these types of image-wise exposure apparatuses, the plurality of the picture elements of the spatial modulation device, which are located in the form of the two-dimensional array, are often raster-scanned in accordance with an image signal, such as a television signal according to the national television system committee (NTSC) standards or the high definition television (HDTV) standards. In such cases, the driving circuit for the spatial modulation device can be formed with a comparatively low cost by utilizing a commercially available integrated circuit for image signal processing, or the like.
It has been proposed that, in cases where a photosensitive material is exposed image-wise by utilizing a spatial modulation device, such as a liquid crystal panel, the so-called "picture element shifting" technique may be utilized in order to increase the density of picture elements of the formed image and thereby to obtain a high resolution image. As described in, for example, Japanese Unexamined Patent Publication No. 4(1992)-7542, with the picture element shifting technique, the optical relationship between the spatial modulation device and the photosensitive material is changed such that, after exposure dots have been formed on the photosensitive material by the light having passed through the picture elements of the spatial modulation device (e.g., in the cases of the liquid crystal panel, the liquid crystal cells), exposure dots may further be formed between adjacent exposure dots by the light having passed through the picture elements of the spatial modulation device. Also, the image-wise exposure is carried out each time the optical relationship is changed.
As for the image formed by the image-wise exposure, the aforesaid exposure dots constitute the picture elements of the formed image. In this specification, such that the picture elements of the formed image may be discriminated from the picture elements of the spatial modulation device, unless otherwise specified, the picture elements of the formed image are referred to as the exposure dots.
For example, the spatial modulation device may comprise i number of picture elements, which are arrayed along an X direction, and j number of picture elements, which are arrayed along a Y direction. In such cases, if the picture element shifting operation is carried out one time along the X direction and one time along the Y direction, 2i number of exposure dots will be recorded along the X direction on the photosensitive material, and 2j number of exposure dots will be recorded along the Y direction on the photosensitive material. Specifically, in such cases, it is possible to record the same number of exposure dots as that of the exposure dots, which will be recorded when the image-wise exposure is carried out one time by using a spatial modulation device comprising 4(i.times.j) number of picture elements. Therefore, the resolution of the image formed on the photosensitive material can be increased.
In order to change the optical relationship between the spatial modulation device and the photosensitive material in the manner described above, the spatial modulation device may be moved. Alternatively, the photosensitive material may be moved. As another alternative, an optical member located between the spatial modulation device and the photosensitive material may be moved.
Ordinarily, in the cases of the spatial modulation device, such as the liquid crystal panel, it is difficult to integrate a large number of picture elements at a high density. However, if the aforesaid picture element shifting technique is employed, a high-density image can be formed with the image-wise exposure by using a spatial modulation device, which comprises a comparatively small number of picture elements located at a comparatively low density.
When the aforesaid picture element shifting operation is carried out, if the photosensitive material is left to be exposed to the recording light, the effects of the picture element shifting operation will become small. Therefore, as described in, for example, Japanese Unexamined Patent Publication No. 4(1992)-7542, it is considered to block all of the picture elements of the spatial modulation device, such as the liquid crystal panel, from light and to carry out the picture element shifting operation in this state.
However, heretofore, sufficient consideration has not been given to the control for setting all of the picture elements of the spatial modulation device to the condition blocked from light. Therefore, in cases where the spatial modulation device is set to the condition blocked from light, a long time has heretofore been required for the image-wise exposure.